In general, a time of flight mass spectrometer (TOFMS) accelerates ions by an electric field to a certain level of kinetic energy and injects them into a flight space having a specific flight distance. In the flight space, the ions are separated by their mass-to-charge ratios according to the time of flight (or “flight time”) until they are detected by a detector. The difference in the flight time of two ions having different mass-to-charge ratios is larger as the flight distance is longer. Therefore, it is possible to enhance the mass resolution by making the flight distance longer. However, conventional types of TOFMSs (e.g. a linear type, reflectron type and so on) have physical restrictions (e.g. the limited overall size) that limit their flight distance.
To solve this problem, some of the recently proposed TOFMSs have multi-turn structures. For example, the TOFMS disclosed in Patent Document 1 has an elliptic loop orbit formed by plural toroidal electric sector fields and makes ions repeatedly fly in that orbit multiple times to increase the flight distance. According to this construction, as the ion makes a larger number of turns along the loop orbit, the flight distance increases and the total flight time becomes accordingly longer. Therefore, the mass resolution increases with the increase in the number of turns of the ion. However, the above-described construction has a problem in that an ion having a smaller mass-to-charge ratio and flying at an accordingly higher speed may overtake another ion having a larger mass-to-charge ratio while they are repeatedly flying in the same loop orbit.
To avoid this problem, Patent Document 2 proposed a TOFMS, in which ions do not repeatedly fly in the same loop orbit but follow a spiral flight path, with their orbits gradually shifting at every turn. This TOFMS includes six pieces of electric sector fields arranged to form a hexagonal flight space through which ions can circuit. It also has a deflecting electric field located between a pair of neighboring electric sector fields. When an ion passes through one of the deflecting electric fields, the electric field shifts the ion in the axial direction of the electric sector field. While the ion is flying through the spiral path, its point of arrival gradually changes along the axial direction of the electric sector fields. Therefore, it is possible to appropriately determine the release point of each ion within a electric sector field so that the ion makes a desired number of turns before it reaches the detector.
To shift the flight path of the ions in the axial direction of the electric sector fields, the above-described mechanism needs multiple pairs of parallel plate electrodes to respectively create a deflecting electric field for each turn of the ions. This means that it requires N−1 pairs of parallel plate electrodes if the ions should turn N times. Such a construction becomes more complex as the number of turns N is increased in order to make the flight path longer. One possible method for simplifying the construction is to employ only one pair of parallel plate electrodes for creating a deflecting electric field that is shared by all the levels of the flight path. However, this construction cannot produce an adequate strength of electric field whose equipotential lines are uniformly distributed across the flight space. As a result, the ions can not follow the ideal deflection path and the performance deteriorates.
[Patent Document 1] Unexamined Japanese Patent Publication No. H11-195398
[Patent Document 2] Unexamined Japanese Patent Publication No. 2003-86129
To solve the above-described problem, the present invention intends to provide a time of flight mass spectrometer having a loop-shaped flight space formed by plural pieces of electric sector fields, which has a simple structure and yet ensures a high level of mass-separation performance by deflecting ions in an appropriate way.